During malaria infection the host responds to a variety of parasite Ags, but the location, number and specificity of the Ags involved have been dificult to determine. Thus during the last year we have used monoclonal antibodies (Abs) to study the antigenic nature of the rodent malaria parasite P. yoelii. The 18 hybridomas which have been studied in detail were produced by fusing P3-O63-NS/1 myeloma cells with spleen cells from mice infected for 4 to 6 days with lethal or nonlethal strain of P. yoelii. Results show that the host responds to a number of Ags located either on the surface or within the cytoplasm of the parasite. The majority of monoclonal Abs identify Ags possessed by all erythrocytic stages of the parasite, but three bind preferentially to meroziotes, and one is completely stage-specific for merozoites. Two of the 18 monoclonal Abs detect Ags shared by the sporozoite and erythrocytic stages of P. yeolii. Approximately 45% of the Abs detect species-specific Ags; 10% denote Ags confined to related rodent malaria parasites (P. berghiei, P. vinckei, P chabaudi); whereas 45% detect Ags shared among rodent, primate (P. knowlesi, P. falciparum) and avian (P. gallinaceum) malarias. The 18 hybrids identify a minimum of 12 distinct plasmodial Ags. 24-35.